T23D-1631
Strontium isotopic evidence of shifting inflows to Eocene Lake Uinta in the Laramide foreland of Utah
Isotopic records from the Uinta basin in Utah are evidence of an evolving landscape during the early Cenozoic. Combined with studies of provenance and paleoflow, oxygen and carbon isotopic results have recently been interpreted to reflect changes in hydrology and catchment hypsometry as the basin responded to developing relief in the foreland. We now present strontium isotope data from lacustrine limestones indicating significant and rapid (< 1 my) shifts in the source of inflowing surface waters. Provenance of Eocene sediments has been used to argue that water spilling south from an overfilled Lake Gosiute in the Greater Green River basin caused a highstand of the lake in the Piceance Creek basin, which in turn overtopped the Douglas Creek Arch and connected with Lake Uinta in the Uinta basin. The lake highstand was extremely productive, and resulted in the deposition of the rich "Mahogany zone" oil shales. New data shows that the 87Sr/86Sr ratio of lacustrine limestones collected in the Uinta basin is generally low (< 0.7105) for most of the Eocene, but spikes higher (to 0.7122) in samples of the Main Body of the Green River Formation near and within the Mahogany zone. We interpret this data to reflect a period of input of water from Lake Gosiute, where that lake's catchments included exposed basement that was much more radiogenic. The strontium data further supports the interpretation that intraforeland basin development in the central North American Cordillera was largely controlled by shifting drainage patterns as the landscape responded to ongoing Laramide tectonism.
T23D-1632
GIS rock unit and lineament analysis distribution in Northern Chihuahua, Mexico: Cenozoic reactivation of the Mojave-Sonora-Megashear (?)
Regional tectonic and geohydrology research for the State of Chihuahua has led to the analysis of rock unit distribution and their relationship to major lineaments. GIS geology maps from Mexico and the United States were used to extract and create shapefiles of four types of rocks: Cenozoic rhyolites, andesites, basalts and conglomerates. In addition to the generation of these layers, maps, including gravity, were converted into raster and georeferenced to serve as ground-truth. Rhyolite distribution demonstrates two different patterns of the upper volcanic series, a vast continuous region and a smaller area in the north where this region is broken in distinctive patterns. Basalt and conglomerate units terminate or decrease in abundance at major lineaments. Characteristics of these features combine to form linear features. Ages of rocks are pre-Miocene, thus the linear, extensional features breaking them up are younger. A strong N-S component is present interpreted to be associated with the Rio Grande Rift. A provocative observation is the presence of multiple WNW trending features. The Rio Papagochic southeast trending topographic embayment is one part of the evidence. Miocene 18 ma NW trending transtensional faulting has been recognized in west Texas. This trend of faults has also influenced breakaway zones of detachment faults in the Early Miocene in northern Sonora. Layering of these newly recognized concurrences over traces of the proposed Mojave-Sonora-Megashear show a strong coincidence. Is this a reactivation?
T23D-1633
Kinematic Evolution of the North-Tehran Fault (NTF), Alborz Mountains, Iran
The ENE-to NW-striking NTF is an active frontal thrust that delimits the Alborz Mountain range to the south with an up to 2000 m topographic break with respect to the adjacent Tehran plain. Eocene rocks of the Alborz range are thrusted over Neogene and Quaternary sediments of the alluvial Tehran embayment. The fault consists of right- stepping segments and merges to the east with the active Mosha-Fasham strike-slip fault (MFF). The complex tectonic history, involving changes in the direction of SHmax, has resulted in a composite tectonic landscape with inherited topographic and fault-kinematic fingerprints along the NTF. We therefore used a combination of fault-kinematic measurements and geomorphic observations to unravel the temporal tectonic evolution of this fault. Presently, the NTF is virtually inactive, although the tectonically overprinted landforms reflect tectonic activity on longer time scales during the Quaternary. Being located adjacent north of the Tehran megacity, there is thus considerable interest to decipher its youngest tectonic evolution and to better understand the relation with other fault systems. Our fault kinematic study has revealed an early dextral kinematic history for the NTF. Dextral strike-slip and oblique reverse faulting took place during NW-oriented shortening. The overall fault-geometry of the NTF suggests that it has evolved in relation to dextral transpression along the MFF. This early kinematic regime was superseded by NE-oriented shortening, associated with sinistral-oblique thrusting along the fault segments. Fault linkage between the semi-independent ENE-striking NTF-segments and NW-striking thrusts (Emamzadeh Davud Fault [EDF], Purkan Vardij Thrust [PVT], NTF-prolongation) point towards an evolution into a nascent transpressional duplex. In this scenario the NTF segments constitute lateral ramps and the NW-striking faults act as frontal ramps. Topographic residuals, as an expression of high-uplift zones, indicate that the central segment of the NTF, incorporating the EDF was most effective in accommodating oblique convergence during this time. However, subtle knickpoints in the longitudinal river profiles crossing the PVT may indicate a relatively recent transfer of deformation onto this block. The youngest manifestations of deformation along the NTF, however, are left-lateral and normal faulting. This youngest phase of activity is documented by numerous striated and rotated conglomeratic clasts, meter-scale fault gouge zones with shear-sense indicators of oblique normal faulting, and multiple colluvial wedges with drag phenomena. Rupture traces and filled extensional cracks reaching the surface also document the seismogenic nature of these features. Since recent left-lateral transtension is also known from neighboring faults, e.g., the eastern MFF, our observations suggest that this youngest phase of tectonic activity of the NTF is a regional phenomenon, rather than the result of locally-determined geometries.
T23D-1634
The Role of Inherited Strike-Slip Faults in the Growth of the Northern Tian Shan
We document the role of inherited strike-slip faults in the northward propagation of the Tian Shan orogen. The Kungey and Zailiskey Ranges (KZR), located along the border of Kyrgyzstan and Kazakhstan, are cored by the ENE-striking Kemin-Chilik fault. This fault is an inherited structure with >30 km of sinistral displacement of Paleozoic basement terranes. Stratigraphic observations show that prior to Neogene reactivation, this region was overlain by a wedge of fluvial sediment derived from more southerly portions of the Tian Shan. The onset of uplift adjacent to the Kemin-Chilik fault is recorded by a transition from distally derived metamorphic clasts to locally derived grus and a reversal of paleocurrent directions. Upward-coarsening records outward propagation and growth of the KZR. We find that deformation is partitioned into sinistral slip on the Kemin-Chilik fault and reverse-oblique slip on a surrounding network of steeply dipping faults. Structural relief on members of this fault network is well-recorded by deformation of a planar unconformity that separates Neogene strata from Paleozoic basement. This unconformity is preserved as a geomorphic surface up to high elevations of the KZR. Activity of fault systems surrounding the KZR is consistent with the long-term pattern of outward growth. We measured fault slip-rates from displaced fluvial terraces in ten different locations surrounding the KZR. These terraces were correlated to a very well-preserved chronosequence at the Toru-Aygir River in the southern Kungey range. Cosmogenic 10Be dating of this sequences yields terrace ages of 85.6 +/- 7.6 ka, 126.4 +/- 10.8 ka, and 139.9 +/- 6.5 ka. Fault slip rates on range-bounding faults vary from 0.07 +/- 0.02 mm/yr to 0.37 +/- 0.04 mm/yr on thrust faults and 1.1 +/- 0.2 mm/yr to 1.5 +/- 0.2 mm/yr on strike-slip faults. Overall, the growth of the KZR surrounding the Kemin-Chilik fault is suggestive of a mega-flower structure with deformation distributed across a network of simultaneously active strike-slip and oblique-slip faults.
T23D-1635
The Hunt for Surface Rupture From the 1889 Ms 8.3 Chilik Earthquake, Northern Tien Shan, Kyrgyzstan and Kazakhstan
The 1889 Ms 8.3 Chilik earthquake in the Northern Tien Shan Mountains of Kyrgyzstan and Kazakhstan is one of the largest historic intraplate reverse-faulting events. Documentation of slip distribution and fault geometry for major historic earthquakes, such as the Chilik event, provide important data on their source physics, seismotectonics, and hazard. These data also provide insight into potential mechanical interaction with other large regional earthquakes, notably the 1887 Ms 7.3 Verny and 1911 Ms 8.2 Kebin (Chon Kemin) events. Despite the importance of the Chilik event, very little is known about the earthquake's epicenter and the presence of associated ground rupture. Isoseismal's from historic shaking intensity data gathered immediately following the event were used to estimate the event magnitude and suggest an epicentral location in the northern foothills of the Kungey Ala-too range, 80-100 kilometers east-southeast of Almaty. Researchers who have visited this area report no evidence of ground rupture, suggesting that the event either did not rupture the ground surface or that the isoseismal data do not provide a sufficiently focused estimate of epicentral location. We have reanalyzed the shaking intensity data to update the estimate of the epicentral location. During June 2007 field work and in review of aerial photography, we observed extensive east-west trending and morphologically youthful km-long and 1-5 m high fault scarps on the eastern crest of the Kungey Ala-Too Range along the Kygyz-Kazak border north of Tyup. Contemporary reports of the 1911 Kebin earthquake did not document these structures, despite the thorough investigation of rupture to the west and northwest. The scarp lengths and their offsets indicate that they did not form in the M6.6 1978 Djalanash Tyup earthquake. Although located to the south of the isoseismal epicenter for the Chilik earthquake, the scarps observed near the range crest must be considered as possible candidates for Chilik ground rupture.
T23D-1636
Denudation of Actively Growing Mountain Ranges in the Foreland of NE Tibet Inferred From in- Situ Produced Cosmogenic Be-10
At the northeastern edge of the Tibetan Plateau ranges bounded by active thrust faults offer the unique opportunity to study the competing effects of uplift and erosion during the early stages of mountain building. Owing to along- strike variations in relief, slope, and lithology, these ranges are an ideal target for studying the influence of topography, lithology, and active faulting on denudation. Here we report spatially-averaged erosion rates for catchments situated along two of these ranges based on Be-10 concentrations of quartz in stream sediments. The Yumu Shan and the western Long Shou Shan are about 60 km long and their overall shape as well as the presence of wind gaps illustrates their vertical-lateral growth during Plio-Quaternary thrust faulting (Hetzel et al. 2004a). Erosion rates determined so far for 20 small catchments are variable and range from 20 to 550 mm/kyr. The observed variability results from at least three factors: (1) the erosion rate in catchments exposing the same lithology is positively correlated with relief and mean slope, (2) weakly consolidated Cretaceous sediments generally erode faster than low-grade Paleozoic bedrock, and (3) the erosion rate seems to decrease from the centre of the fault-bounded ranges towards their propagating tips. As rates of thrust faulting and rock uplift in the region (600-1200 mm/kyr; Hetzel et al., 2004a, b) exceed the denudation rates, the active growth of mountains and the lateral growth of Tibet has not yet come to rest. References Hetzel, R., Tao, M., Niedermann, S., Strecker, M.R., Ivy-Ochs, S., Kubik, P.W., Gao, B. (2004a). Implications of the fault scaling law for the growth of topography: Mountain ranges in the broken foreland of NE Tibet, Terra Nova 16, 157-162. Hetzel, R., Tao, M., Stokes, S., Niedermann, S., Ivy-Ochs, S., Gao, B., Strecker, M.R., Kubik, P.W. (2004b). Late Pleistocene-Holocene slip rate of the Zhangye thrust (Qilian Shan, China) and implications for the active growth of the northeastern Tibetan Plateau, Tectonics 23 (6), TC6006, doi:10.1029/2004TC001653.
T23D-1637
Locating the boundary of the North and South Lhasa Block: Eclogites within the Gandese belt, Southern Tibet
A nearly east-west trending eclogite belt, several hundred meters wide and tens of miles long, occur from Sumdo to Jiaxing, East of Lhasa. Field, petrography, and geochemical observations show that: (1) though affected by the late stage of fluid/magma infiltration, most of eclogites within this belt have preserved N-MORB type trace element geochemistry; (2) thin layers of eclogite are sandwiched between quartzite layers, which mimic the sheet-dike structure of a typical ophiolite; (3) fresh eclogites consist of garnet (Gt), omphacite (Omph), epidote (Epi), amphibole (Am), Phengite (Phen), rutile (Rt), and minor apatite (Ap), zircon (Zr), and iron oxides; (4) highly retrograde eclogites consist of garnet, amphibole, plagioclase (Pl), paragonite (Para), quartz (Q), rutile, and minor apatite and zircon. Relict omphacites only can be seen in the core of large hornblende or as inclusions within garnet. Two types of garnet occur in these eclogites: garnets with a lot of inclusions do not show any chemical zoning, whereas garnets free of inclusions show distinctive zoning. Such zonings are characterized by sharp increase in Fe, decrease in Mg, but nearly constant Ca from core to rime, which suggest a nearly isobaric heating during their exhumation. Therefore, emplacement of the Mesozoic Gandese granitic batholith might have been responsible for the latest exhumation of the Sumdo eclogite. Omphacites in the matrix also show distinct zoning with Na2O decrease from 7.0 to 4.0 wt% toward their rims. However, omphacite cores have similar compositions to those of tiny garnet-hosted inclusions. Preliminary data indicate that metamorphic reactions in these eclogites are represented by prograde Gt-1 + Amph + Epi + Q + Rt + Ap + Zr, Gt-1 + Amph + Omph-1 + Epi + Rt + Ap + Zr, and peak Gt + Omph-1 + Phen + Rt + Ap + Zr, and retrograde Gt-1 + Omph-2 + Amph + Epi + Phen + Ap + Zr, and Gt-2 + Amph + Epi + Para. The nature of the Mesozoic-Early Cenozoic granotoids within the Lhasa Block show substantial N-S variations, suggesting a fundamental difference in the lithospheric structure across the Lhasa Block. This eclogite belt may represent such a plate boundary of the North and South Lhasa Block.
T23D-1638
U-Pb Detrital Zircon Geochronology and Implications: the Mesozoic southern Ordos Basin, North Central China
Two deformation belts — the Qinling orogenic belt (QOB) to the south and the Western Liupanshan thrust belt (WLTB) to the west, including the Qilian-Qaidam terrane (QQT) — control the evolution of the southern Ordos Basin during early Mesozoic time. U-Pb detrital zircon geochronology is used to identify provenance, reconstruct paleogeography, and document the relative timing of basin margin deformation. Two correlated strata same age samples of the Middle to Late Triassic Yanchang Fm. were collected from the southern and southwestern basin margin respectively. In total 258 zircon grains were analyzed using LA-ICP-MS. Three major age populations — 240–490 Ma, 1.8-2.0 Ga, and 2.2-2.8 Ga — characterize the detrital zircon grains of the Yanchang Fm. Regional geochronology studies show that the two oldest age groups match ages of basement rocks found in the underlying North China block. The younger age group can be subdivided into three distinctive groups: 240-300 Ma, 300-400, and 400-490 Ma. The youngest age group matches ages exposed to the west in the QQT, whereas the other two match grains from southern QOB source. In all samples detrital zircon from the WLTB source area overwhelms the QOB source, regardless of its proximity to either basin margin. This result suggests that either the source rock with the youngest zircon grains (e.g. 240- 490 Ma) from the QOB to the south have been eroded away and no longer existed, or simply the QOB, even during deformation, was never the dominant source of detrital zircon to the southern Ordos Basin. Probably some drainage systems far from the west combine with some local tributary systems from the south control the sedimentation in the south Ordos Basin during the middle to Late Triassic time. This is surprising in that most studies suggest that the QOB was a longer lived and more dominant tectonic feature in this part of China during early Mesozoic time. Furthermore, it also indicates that: 1) deformation in the QOB and WLTB began in Middle Triassic time in this area; and 2) the southern source area barely contributed sediment by Late Triassic time.
T23D-1639
Structural Geology of the Western Sichuan Basin, China: Implications for the Growth of the Tibetan Plateau
The Longmen Shan mountain range, located between the Tibetan plateau to the west and the Sichuan basin to the east, exhibits greater relief than anywhere else in Tibet. However, geodetic surveys measure little to no shortening across the range front and geologic mapping suggests that there are few if any active faults in the area. Theories for the creation and existence of the Longmen Shan suggest that either 1. The mountains were uplifted through brittle faulting some time ago and have been relatively inactive since then, possibly since the late Miocene (~5 Ma); or 2. The relief across the mountain range is maintained by dynamic pressure from lower- crustal flow. The first theory requires that topography is caused by crustal shortening through faulting and folding, while the second suggests that shortening and topography are decoupled. We construct balanced cross- sections across the Sichuan basin using industry seismic reflection data. We measure shortening along these cross-sections, and note a large increase in shortening beneath the Longmen Shan. Our results show that shortening and topography are correlated. This suggests that a simple lower-crustal flow model is unlikely, and if lower-crustal flow is true, it may drive some sort of detachment system coupling the uplift of the crust to shortening.
T23D-1640
A Climatic Trigger for Enhanced Late Cenozoic Exhumation of the Chinese Pamir?
In the Chinese Pamir mountains at the western end of the Tibetan Plateau, two high peaks (Kongur Shan at 7719 m and Muztagh Ata at 7546 m), rise >4000 m above the plateau, representing an area of significant anomalous topography. These mountains are situated below the Kongur detachment, correspond with domal structures cored by gneisses exhumed since 6-8 Ma from up to 27 km depth (Robinson et al., GSAB, 2004). Exhumation of these domal structures is undoubtedly related to movement along the Kongur detachment fault. However, the locally more rapid exhumation rates in the area of these two domes, their association with extensive glaciers with headwalls up to 2 km high, and with deeply incised river gorges immediately to the north and south of the massifs point to additional exhumation of the domes driven by positive feedback between focused fluvial and glacial erosion and thermal and mechanical weakening of the crust. It is possible that glaciation is a passive result of tectonic exhumation, in which tectonically produced high peaks lead to greater orographic precipitation, resulting in the development of larger glaciers, or that the peaks are the location of a "tectonic aneurysm," driven to greater exhumation and height by focused glacial erosion. New apatite fission track (AFT) and argon data from both the footwall and hanging wall along the length of the detachment provide a more detailed view of the latest Cenozoic cooling history. In the north, along the Muji segment, and in the middle, near Kongur Shan, hanging wall AFT ages are 4-6 Ma. Footwall AFT ages in the Muji segment and in the south, along the upper Yarkand river are ca. 3 Ma. However, the footwall of the western flank of Kongur Shan yields AFT ages between 1 and 2 Ma, very similar to our own and published argon ages on biotite and muscovite, the oldest of which is 2.5 Ma (muscovite). Published argon MDD modeling of three feldspars from the northern flank of Kongur Shan show a significant increase in cooling rate at ca. 2 Ma. Our samples from closer to the detachment suggest that accelerated cooling started slightly earlier. The preliminary data implies that this acceleration in exhumation rate at ca. 3 Ma was roughly synchronous over ca. 150 km along the length of the detachment. The timing and spatial distribution of this event is more readily explained as a consequence of enhanced erosion due to climate change rather than a tectonic effect due to a change in the regional stress field. The largest magnitude and highest rate of exhumation is associated with the most significant , glaciated topography, suggesting a feedback between focused surface processes and exhumation.
T23D-1641
Investigating the Tectonic Aneurysm Model: Measuring Slip Rates Along the Kongur Detachment Fault in the Chinese Pamir
The Kongur Shan and Muztagh Ata massifs in the Chinese Pamir have experienced recent exhumation at a rate not readily explained by the crustal shortening associated with the Indo-Asian collision. Given the extensive glaciation of the peaks and the deep exhumation compared to the surrounding region, it has been proposed that Kongur Shan and Muztagh Ata are undergoing a ‘tectonic aneurysm', a process in which the positive feedback between focused erosion and exhumation of the upper crust results in lower crustal flow into the area and increased relief. The exhumation of these peaks is primarily accommodated by the Kongur detachment fault, in addition to several lesser normal faults trending sub-parallel to the main detachment. Previous studies have used thermochronology to determine the long-term exhumation history of the footwall, and have identified a possible increase in cooling rate at ~2 Ma near the peaks which has not been experienced by the rest of the range. If a tectonic aneurysm is taking place, recent fault slip rates should be higher close to the massifs and should increase through time. By mapping regional stream terraces displaced by this fault system with the aid of a handheld GPS receiver/recorder, this study aims to quantify the total recent vertical displacement at several points along the detachment. To establish slip rates, age control must be applied to offset surfaces, thereby yielding a maximum age for the tectonic event offsetting the surface and a minimum slip rate. A previously established age of 39+/-5 ka to 58+/-15 ka for the uppermost terrace surfaces near Kongur Shan, obtained in a previous study using Cosmogenic Radionuclide surface exposure dating, is used here. The fault system was mapped at three locations NW of Kongur Shan (at various distances from the peak): Zouba Valley (80km), Wulu Valley (65km), and Kalagile (40km). Near the north end of the Kongur detachment, displacement across the Wulu and Zouba faults sums to 0.53-1.14 m/kyr. The slip rate at Kalagile, nearer the massif, is 0.82-1.76 m/kyr. This increase in slip rate near Kongur Shan is consistent with the occurrence of a tectonic aneurysm.
T23D-1642
Cenozoic Right-slip Faulting Along the East Flank of the Pamirs, NW China: Implications for the Kinematic Evolution of a Major Salient in the Northern Margin of the Himalayan-Tibetan Orogen
The Pamir salient defines the western end of the Himalayan-Tibetan orogen in China, Tajikistan, and Kyrgyzstan. The leading edge of this salient has overthrust the Tarim-Tajik basin to the north by ~300 km along a late Cenozoic, south-dipping intracontinental subduction zone beneath the North Pamir. Its eastern flank trends NW- SE, and lies between north-directed thrust belts in the North Pamir, to the west, and the Western Kunlun Shan, to the east. New 1:100,000-scale geological mapping, detailed structural measurements, and analysis of mesoscale structures along a 40 km-long transect along the east-flowing Yarkand River document the tectonic evolution of the this flank of the Pamir salient. The study area is cut by a set of four, NW-striking, steeply dipping, brittle faults with right-lateral slip, as indicated by brittle microstructures and asymmetric, outcrop- to map-scale folds. Panels of Phanerozoic strata that lie between these faults are deformed by en échelon folds with axes that trend more westerly than the adjacent faults, consistent with dextral transpression. Synthesis of these new results with previous regional geologic mapping suggests that the fault system described here extends for a total of ~350 km along strike, forming a structural system that defines the eastern flank of the Pamir salient and is here called the Kashgar-Yecheng transfer system. Transpressional right-slip along the Kashgar-Yecheng transfer system appears to have accommodated late Cenozoic separation of the North Pamir from the Western Kunlun Shan during south-directed intracontinental subduction beneath the leading edge of the Pamir salient. Correlation of major faults suggests total slip along the Kashgar-Yecheng transfer system of up to ~280 km. When combined with previous sedimentologic, stratigraphic, and thermochronologic data indicating deformation along the east flank of the Pamir started between the Late Eocene to Early Miocene, this offset estimate implies long term slip rates of 7-15 mm/yr along the Kashgar-Yecheng transfer system. Our results imply that the first- order structures on the western and eastern flanks of the Pamir are strongly asymmetric: in contrast to the transpressional right-slip transfer faulting we find on the east side, previous work has shown that deformation in the west was accommodated by northwest-directed radial thrusting and associated anti-clockwise vertical axis rotation of the Pamir over the eastern margin of the Tajik basin.
T23D-1643
Slow steady exhumation of the high elevation Deosai Plateau (Northern Pakistan Himalaya) since 40 Ma
Mountain ranges of the north-western Himalaya in Pakistan show strongly contrasting relief, opposing steep, deeply incised topography with extremely high peaks such as the Karakorum Range and Nanga Parbat Haramosh Massif (NPHM), to high-altitude, low-relief areas such as the Deosai Plateau located between the Karakorum and NPHM and the Tso-Morari Massif in Eastern Ladakh. In contrast, mean elevations of the different mountain ranges are comparable, the Deosai Plateau being on average even slightly higher than the adjacent NPHM. The aim of this study is to quantify the exhumation history the Deosai Plateau, in order to understand how to build such a high-altitude, low-relief plateau and how to preserve it over million-year timescales. Here, we report the first low-temperature thermochronologic data from the Deosai Plateau, to compare its exhumation history to that of the surrounding massifs. Apatite Fission Track (AFT) ages reported in the literature from the NPHM and Karakorum are extremely young (<1 Ma for the NPHM and between <1 and 7 Ma in Karakorum) implying exhumation rates >1 km/Ma. In contrast, our AFT ages from the Deosai Plateau are 15-27 Ma; an order of magnitude older than those of the surrounding massifs. Zircon and apatite (U-Th)/He ages (measured at U of Arizona, HeDWaAZ program) range from 23-45 Ma, and 12-15 Ma, respectively. Modeling the combined AFT and He ages requires continuous and very slow long term cooling rates (around 4 ° C/Ma), consistent with an exhumation rate of about 0.15 km/Ma for typical geothermal gradients. Our data thus suggest steady slow unroofing of the Deosai Plateau since at least 40 Ma. A clear link between cooling age patterns and the geomorphology is also evident, with strongly incised, high-relief massifs showing exhumation at rates an order of magnitude faster than the low-relief plateau. AFT and ZFT ages similar to our data have been reported from the Tso Morari massif further east, characterised by similar high-elevation low-relief morphology. These morphologic zones cross the Indus and Shyok Suture zones and are apparently independent of these structures. We suggest that these isolated plateau remnants can be considered as pieces of an early, highly elevated south Tibet Plateau that were isolated from it by more recent Karakorum fault movement and associated exhumation.
T23D-1644
Rigid Basement and the Evolution of the Pakistani Convergent Margin
In Pakistan, along the western edge of the Indian-Eurasian collision there are a series of fold-and-thrust belts that have highly variable strikes and shortening directions with respect to the local relative plate motion. Much of the complexity in the deformation of this margin can easily be explained by the shape, location, and long-term motion of a fragment of relatively rigid oceanic lithosphere that is believed to underlie the Katawaz Basin. In particular, the deformation that has formed the Sulaiman Range and Lobe is a direct consequence of the Katawaz Basin's over all higher strength. The presence of deformed sedimentary strata in the basin comparable to those presently found in the Indus delta are indicative of the basins long-term motion parallel to the Chaman fault zone. In Pakistan, the transition in the strike and shortening directions occurs over a short distance compared to the width of the fold-belts and the length of the margin. We present a series of analog models along with detailed quantitative analysis that we compare to the observed deformation as indicated by both geologic and geophysical data. By quantitatively distinguishing the style and magnitude of deformation in each of a variety of analog experiments we are able to evaluate the viability of various alternative models that have been proposed for fold- belt formation and evolution of the Pakistani margin, including our favored model. The model that best fits the geological and geophysical evidence suggests that the complexity of the Pakistani margin is a result of the long- term northeastward migration of the Katawaz basin along the curving trend of the Chaman fault zone. The vertically integrated mechanical strength of the Katawaz basin allows it to act as a strong ‘backstop' that has relative motion to both stable India and stable Eurasia. This northeastward motion and the resulting clockwise rotation of the Katawaz ‘block' during the margin's development can explain the location and intensity of eastward shortening and the pattern of seismicity observed in the Sulaiman Lobe and range, the formation of Kingri Fault and Sibi reentrant.
T23D-1645
Spatial and Temporal Variations in Exhumation across the NW-Himalaya
Exhumation in mountain belts is temporally and spatially variable over long time scales (> 106 yr). The topography is a result of changes in both the location of deformation (mountain growth) and erosive climate (mountain decay). However, many studies are limited in the extent to which spatial variations in exhumation can be quantified due to the limited geographic extent over which samples are collected. We investigated spatial and temporal variations in exhumation and deformation across a 150x200 km region of the NW Himalaya, India. 25 new and 168 previously published apatite and zircon fission track and muscovite 40Ar/39Ar ages are integrate with a 1D model to quantify rates and timing of deformation and exhumation/erosion along strike of several major structures in the Lesser and Higher Himalaya. The model solves the advection-diffusion equation and predicts thermochronometer ages for variations in exhumation/erosion rates, thermophysical properties, and thermal gradients. The new and previously published apatite fission track ages range from 10 to 0.6 Ma. The sample area is characterized by 0.7-5 km-high-topography, high relief (about 4 km), and large (<0.3 to 2.5 m/yr) precipitation gradients. All the thermochronometer data indicate large temporal and spatial variations in exhumation. Erosion and exhumation rates are inversely correlated with local relief and specific stream power, but not with modern precipitation gradients. Results from thermokinematic modeling of all thermochronometer systems indicate: (1) High Himalaya exhumation rates were <0.5-0.8 between about 15 and 4 Ma and increased to 1.5-3 mm/yr after 4 Ma. (2) In the Lesser Himalaya exhumation rates are 1mm/yr over the last 10 Ma. The previous temporal variations in erosion in the Greater Himalaya are spatially consistent along 200 km of strike, regardless of structural variations. This suggests that tectonics as well as erosion, has controlled the exhumational evolution of this region. The correlation between erosion/exhumation rates derived from AFT ages and present day specific stream power suggest that the location of deformation and rock uplift has been consistent for the last 4Ma. Thus the High Himalaya has been exhumed rapidly since then, however ZFT and muscovite 40Ar/39Ar do not provide additional constrains, if exhumation have been consistent for longer time.
T23D-1646
Rates of Fluvial Incision and Exhumation in an Active Mountain Belt, Lahul Himalaya, Northern India
The Lahul Himalaya of northern India provides an excellent natural laboratory to define rates of denudation and exhumation in an active mountain belt because its Quaternary history — derived from well-preserved and dated successions of landforms and sediments — is well established. Building on previous tectonic and geomorphic studies, terrestrial cosmogenic nuclide (TCN) 10Be surface exposure dating and apatite (U-Th)/He (AHe) thermochronology was applied along the Chandra Valley to quantify rates of fluvial incision and exhumation. TNC ages on successive strath treads at four locations along the Chandra River and one of its tributary rivers, between Batal and Khoksar, range from 0.56 to 5.36 ka, yielding post-glacial (after 10 ka) rates of fluvial incision of 1.5 to 13.5 mm/a. The highest rates are associated with straths along a narrow canyon-like stretch of the main drainage (Chandra River) and an associated tributary valley. These are an order of magnitude greater than those of gentler, broader alluviated reaches of the Chandra. The highest rates may suggest a change in uplift or landscape incision modulated by glacial fluctuation. AHe ages from leucogranite exposed near the Chandra River and at higher elevations in steep, deeply gorged, glaciated side valleys range between 1.46 and 2.51 Ma, marking a pulse of rapid cooling and denudation during the Late Pliocene-Quaternary. These TNC and AHe data suggest enhanced denudation of the region during the Quaternary and provide some of the first quantitative data for rates of landscape evolution in the Lahul Himalaya of northern India.
T23D-1647
Incipient graben formation in the NW Himalaya (Himachal Pradesh, India)
GPS data and regional geological compilations show that the highest sectors of the Higher Himalaya are currently undergoing extension. This has been mainly explained with radial extension along the curvature of the Himalayan arc. However, close inspection of neotectonic extension phenomena using high-resolution satellite imagery from the upper Sutlej Valley of the NW Himalaya (Himachal Pradesh, India) suggests that the recent extension direction is rather oriented E-W, and that N-S graben-bounding normal faults in southern Tibet may propagate southward. This assessment is further corroborated by our new detailed structural field investigations and fault kinematic analysis. Our studies reveal that these normal faults cut all previously generated structures, documenting the recent activity of this fault generation. The average offset of these N-S striking, steeply dipping normal faults is in the cm to dm range for a single fault plane. However, since these brittle faults are part of a densely spaced network, the cumulative offset must be significant. Fault kinematic analysis of slickensides demonstrates that these structures are an integral part of a N-S oriented zone of diffuse E-W extension between the Leo Pargil gneiss dome in the north and the Garhwal Himalaya in the south. This E-W extension is also compatible with the orientation of T-axes of earthquake focal mechanisms obtained in the same region. Although different data sets document ongoing E-W extension, the reason for this phenomenon is not well understood. Despite the fact that some of the normal faults cut Quaternary and Tertiary units, and earthquakes occur within the realm of the extensional Leo Pargil gneiss dome, the ubiquitous occurrence of these structures within the Higher Himalaya suggests an orogen-wide origin, independent of extensional processes associated with dome formation. Alternatively, we therefore interpret the neotectonic extensional evolution of this region to reflect a very early state of a graben system propagating southward that is influenced by the E-W tensional stress regime that governs the southern Tibetan Plateau.
T23D-1648
Geometry and Kinematics of Quaternary Fault and Fold Growth Along the Northeastern Himalayan Front: Implications for Himalayan Drainage Development and Active Expansion of the Himalayan arc.
Understanding active mountain building can provide a guide to determine the processes responsible for the formation of older orogens. With regard to the Himalaya, early research efforts have mostly focused on Tertiary structures located within the mountain range such as the Main Central Thrust (MCT). Only minimal effort, all conducted in the western and central Himalaya, was devoted to investigation of the Main Frontal Thrust Zone (MFTZ), the youngest Himalayan fault system that is actively uplifting and expanding the Himalayan range. Our study addresses this issue by integrating field- and satellite-based structural data at several sites along the eastern Himalayan front (Arunachal) in NE India. On a regional scale, the interaction between the MFTZ and the eastern Himalayan drainage systems is most prominently displayed. Specifically, folds are aligned in an en echelon fashion at about 20-30 degrees from the main trace of the Himalayan frontal thrust that defines a continuous topographic front. The en echelon folds in map view indicate a left-slip shear across the eastern segment of the Himalayan front. All the folds are east-plunging and are associated with eastward deflection of generally south-flowing Himalayan rivers. We focused our study at one site where the Main Frontal Thrust is expressed by the Bhalukpong thrust and the growing fold is represented by the southeast-plunging ~25 km- long Balipari anticline, across which the generally south-flowing Kameng River is deflected to the east around its eastern nose. Associated with the river deflection are several levels of tilted and uplifted terraces. The Bhalukpong thrust ~10 km north of the anticline juxtaposes a recumbent fold in the Miocene Dafla Formation over Quaternary gravels. Although the thrust bounds several levels of Quaternary terraces in its hanging wall, the fault trace itself is covered by a young fluvial terrane, suggesting that the fault has not been active since its deposition. Field mapping and a total-station survey revealed three major terrace levels across the contractional structures. We also use 29 14C samples to date the terraces, among which the age of the T2 riser on the south side of the anticline is best constrained. Three samples from T2 were collected across the south flank of the anticline: two from our best total-station-survey sites at the northwest side of the fold and one at the nose of the anticline, which yield ages between 883+/- 38 yrs BP and 922 +/- 11 years BP. Abandonment of T1 above the Bhalukpong thrust was constrained by a single charcoal age at 618 +/- 37 yrs BP. With respect to end- member models for the Himalayan arc growth emphasizing cross-sectional evolution (i.e., fault-propagation folding in the western Himalayan front and fault-bend folding in the central Himalaya), our work points to the importance of lateral growth of contractional structures, alternating motion among several contractional structures in the MFTZ, and role of orogen-parallel shear at the ends of the Himalayan arc.
T23D-1649
Preliminary Stratigraphy and Structure of the Lesser Himalayan Portion of the Himalayan Fold-Thrust Belt, Eastern Bhutan
Documenting the magnitude and geometry of shortening across the Himalayan orogen is a critical step in testing competing hypotheses of systematic variation along strike due to possible changes in convergence magnitude, observed changes in the width of the Tibetan Plateau, or the possible influence of precipitation gradients or stratigraphic variation on strain accommodation. While balanced cross sections allow for shortening estimates in the western and central parts of the orogen, the eastern portion remains understudied. Thus, the acquisition of data here is necessary to document along-strike changes within the Himalayan orogen. We present new mapping and U-Pb detrital zircon (DZ) ages within Lesser Himalayan (LH) rocks of eastern Bhutan, between the Main Central Thrust (MCT) and Main Boundary Thrust (MBT), and describe the first-order stratigraphy of this portion of the northern Indian passive margin. We use the stratigraphic data to construct balanced cross- sections that illustrate the geometry of this portion of the fold-thrust belt, and allow for preliminary estimates of minimum shortening. Preliminary U-Pb DZ analyses divide the LH section into 2 groups, 1) lower LH units, with the youngest DZ peak at ~1.8 Ga, and 2) younger, upper LH units with DZ spectra between ~1000-500 Ma. The oldest zircons (1.8-1.9 Ga peak) are in the Shumar and Daling Formations; this signature is characteristic of correlative LH units along strike in Nepal. Younger LH zircons were found in the Baxa Formation in the frontal portion of the fold-thrust belt (as young as ~500 Ma), and within 2 samples collected just beneath the MCT (~1000-500 Ma), in rocks mapped in stratigraphic contact directly above the Daling Formation. These data indicate that Proterozoic LH units identified in Nepal are continuous along strike, but argue for the discontinuous presence of important Paleozoic basins. Also included in the upper LH formations are the Diuri Formation (predominantly diamictite), and the sandstone, shale and coal of the Gondwana Sequence. Above the LH strata are up to 6 km of foreland basin deposits of the Neogene Siwalik Group. Major structural features within LH units in eastern Bhutan include 1) a duplex within upper LH units, immediately north of the MBT, and 2) a northern duplex of lower LH units. We propose that the Shumar Thrust, which separates the two domains, is the roof thrust of the upper LH duplex while the MBT is the floor thrust. A preliminary, composite balanced cross-section of the fold-thrust-belt estimates minimum shortening at 326 km, or 72%. This estimate is much less than those in Nepal (~500-700 km), indicating significant along-strike variation.
T23D-1650
Can we Detect Recent Activation of the Main Central Thrust Zone from Apatite Fission Track Data and Numerical Modelling?
We study the recent dynamics of the Himalayan orogen in central Nepal with the specific goal of quantifying the onset of activity and the deformation history recorded by the different major thrusts. Here, we focus on the possible reactivation of the footwall of the MCT, which is marked by a strong topographic transition in the Nepal Himalaya. This transition is too sharp to be interpreted as a simple consequence of erosion and tectonic mechanisms, such as out of sequence thrusting (Hodges et al., 2004) or underplating over a major crustal ramp (Bollinger et al., 2006) have been suggested. We report 14 new apatite fission-track (AFT) ages collected along a north - south transect from Langtang in the High Himalaya to the Terai Plain. AFT ages are consistently young (< 3 My) all along the N-S transect in the MCT zone and older (4 to 6 My) in the southern part of the Lesser Himalaya. The topographic transition does not correspond to a sharp AFT age transition. We perform numerical thermal-kinematic modelling, using a modified version of the PECUBE code (e.g. Braun, 2002), to quantify the respective roles of underplating and out-of-sequence thrusting in the shortening of this part of the Himalayan range. In these initial models, we assume steady state between tectonic accretion and surface erosion. The current dataset cannot discriminate the two models: although out-of-sequence thrusting and reactivation of the MCT shear zone are consistent with the data, they do not require such a mechanism. The presence of a crustal ramp below the topographic transition exerts the primary control on age patterns. Therefore, the question of possible out-of-sequence reactivation appears secondary in face of the current data set.
T23D-1651
Exhumation and Uplift of the Shillong Plateau and its Influence on the Eastern Himalayas: new Constraints From Apatite and Zircon (U-Th-[Sm])/He and Apatite Fission Track Analyses
The Shillong plateau is the only raised topography in the foreland of the Himalayas. Located on the trajectory of the Indian Summer Monsoon (ISM), the plateau perturbs the regional distribution of precipitation. As such the Shillong plateau-eastern Himalaya-ISM is a unique system to quantify the couplings between climate, tectonics and erosion. A change in long-term erosion rates along-strike the Bhutan Himalaya was recently attributed to a climatic modulation due to the uplift of the Shillong plateau. To test this interpretation, it is essential to constrain the timing and rate at which the plateau was uplifted and the amount of partitioning of the India-Asia convergence into the plateau. We used apatite and zircon (U-Th-[Sm])/He and apatite fission-track analyses to unravel the thermal histories of thirteen basement samples collected along a N-S transect across the central Shillong plateau. We find that 1) the exhumation of the plateau began at least 9-15 Ma ago, 2) its surface uplift was chronologically decoupled from its exhumation and started after ~3-4 Ma at rates of 0.4-0.53 mm/yr, 3) the long- term horizontal shortening rate accommodated by the plateau is 0.65-2.3 mm/yr, which represents only 10-15 percent of the India-Asia convergence rate. The uplift of the Shillong plateau did not significantly modify the rock uplift rate in the Bhutan Himalaya, which is consistent with the hypothesis of climatic modulation of the Pliocene erosion, tectonic and landscape evolution previously documented along this orogenic front.
T23D-1652
Drainage Analysis and Fluvial Terrace Reconstruction: Assessing Blind Thrust Hazards, Montecitos Anticline, Mendoza, Argentina
Blind thrust faults pose a problem for detailed tectonic and hazard assessment because their lack of surface rupture and sometimes subtle geomorphic expression makes traditional fault recognition and analysis difficult. It is critical to understand and prepare for earthquakes on these faults. The Andean Precordillera between 31° and 33° has experienced several devastating earthquakes in the last century, and geodetic studies indicate that this area has a long-term shortening rate of ~5mm/yr. One of the growing anticlines in this region that partially accommodates this shortening is the Montecito anticline, a fault propagation fold above a blind thrust fault. The Montecito anticline is located just north of Mendoza in the Andean Piedmont and is approximately 7 km long and 2 km wide. Drainage diversions, changes in river sinuosity and deformation of Holocene alluvial fan sediments all indicate that the anticline has been active during the Quaternary. We have used a combination of geomorphic techniques, including drainage analysis and fluvial terrace reconstruction, dating and GPS analysis to understand the geometry and development of the Montecito Anticline. By analyzing differential river profiles of the two transverse drainages we have been able to quantify localized uplift and examine changes in stream gradient across the structure. Stream diversion studies show the average regional flow direction, and the amount and direction of stream deviation from this average in the vicinity of the anticline. 11 river terraces formed along these drainages have been examined using kinematic GPS and ArcGIS for evidence of warping and rotation. Cosmogenic radionuclide dating of 5 of these terraces will constrain uplift rates along the axis. The combination of these techniques has yielded considerable information about the uplift and deformation history of this blind thrust fault. By analyzing these types of faults and continuing to quantify the seismic risk that they pose to nearby population centers we hope to contribute to a greater understanding of earthquake hazards in Mendoza area, as well as to refine geomorphic techniques by which blind thrust faults can be recognized and assessed in other locations.
T23D-1653
Asymmetrical Growth of Footwall Topography in the Cordillera Blanca, Peru: Implications for Normal Fault Control on Landscape Evolution
Morphometric parameters and hypsometry of the >5-km-high Cordillera Blanca in Peru provide insights into the topographic evolution of the uplifted footwall of an active, high-magnitude-slip, low-angle normal fault within a contractional orogenic belt. The modern tectonic activity, mappable fault trace, high relief, and uniform footwall lithology make the Cordillera Blanca an ideal locality to test models for the evolution of topography related to normal faulting. Most models for normal fault growth suggest a tectonic and geomorphic symmetry in which maximum slip and maximum surface uplift occur along the central segments of the fault, with net slip, footwall uplift, and hangingwall subsidence diminishing along strike toward the fault tips. A digital elevation model (DEM) of the Cordillera Blanca permits extraction of important geomorphic metrics for footwall transverse drainages and the hanging-wall axial river, including hypsometry, drainage area, drainage length, relief, channel gradient, surface slope, aspect ratio, and longitudinal profile. These data reveal a pronounced asymmetry in which footwall relief, exposed fault relief, channel gradient, and surface slope are greatest along the northern fault segment and become systematically lower southward along strike. In contrast to most models for normal fault growth, largely based on late Cenozoic systems in the Basin and Range province of the western United States, the Cordillera Blanca demonstrates that abrupt and significant along-strike variations in displacement and footwall uplift are possible in major normal fault systems. The causes of an asymmetric distribution of fault slip and footwall topography in the Cordillera Blanca may be related to: (a) mechanical coupling between the subducting flat Nazca slab and the overriding South American plate; (b) gravitational collapse of overthickened crust along reactivated thrust faults; (c) climatic processes linked to the effect of glacial erosion in the footwall and/or fluvial incision in the hanging wall; or (d) a combination of rapid erosional processes and batholith intrusion that removed the strong upper crust and thermally weakened the remaining crust, allowing extension and exhumation along the Cordillera Blanca normal fault.
T23D-1654
The Algerian Margin: an Example of a Reactivation in Compression of a Complex Cenozoic Passive Margin
The Western Mediterranean underwent a complex Cenozoic history involving subduction of the Tethys Ocean as well as subduction roll-back and associated opening of back-arc basins. During the Oligo-Miocene, the subduction roll-back to the south led to the collision of the Kabylies into the African plate, but subduction continued towards west, causing the Alboran slab to migrate towards the Gibraltar Arc. Northern Africa is at the southern border of this system and is therefore a major study area in the context of slow convergent plates to study the reactivation in compression of a Cenozoic passive margin but also the records of past geodynamic processes. This work aims to characterize the multi-scale structure of the offshore Algerian margin, based on the MARADJA'03 and MARADJA2/SAMRA'05 cruises data (multibeam bathymetry, seismic-reflection, side-scan sonar, backscattering, CHIRP, gravimetry). Tectonic (geomorphology, folds, faults) records reveal large recent and active structures as well as the geological inheritance of the margin. In western Algeria, slab roll-back is likely to have been accompanied by lithospheric tearing (STEP fault) as it has been modelled at a regional scale (Govers and Wortel, 2005): we provide first evidence for the presence of such structure(s) offshore Algeria. The geodynamical conditions have now changed, and we are facing new types of structures. Two main tectonic styles are identified: reverse to the centre and east; and strike-slip to the west. In Central Algeria, the compressional structures are active blind thrusts (Plio-Quaternary) verging to the north (opposite to pre-existing features) expressed as asymmetrical folds, sub-perpendicular to the convergence direction and often en echelon. These faults may all trigger M=6-7.5 earthquakes (e.g. Khair al Din fault near Algiers). Among them, the fault associated with the 2003 Boumerdes event (Mw=6.8) would continue to the surface by flats and ramps creating piggy-back basins or rollovers. From our estimate of sub-surface shortening on Quaternary structures, a significant part of the deformation resulting from Africa-Eurasia convergence (about 5 mm/yr at the longitude of Algiers) appears to be accommodated at the foot of the Algerian margin, which could indicate, with the compressional flexure of the deep basin, a future subduction inception.
T23D-1655
Climate and Tectonic Controls on Sedimentation and Incision in the Fiambalá Basin, Northwest Argentina
Global climate change during the Late Pliocene has been credited for a worldwide increase in erosion and sedimentation rate, and a coarsening of clastic sedimentation. This may be a result of lowered sea levels, increased glacial erosion at high altitudes and latitudes, or large climatic fluctuations. However, local structural deformation and regional tectonic uplift may also explain increased sedimentation rates. Clastic sediments preserved within intermontane basins adjacent to plateaus offers a unique record of the timing and pattern of orogenic evolution and its relationship to tectonics and climate. The Pliocene Punaschotter conglomerates of the Fiambalá Basin along the southern margin of the Puna Plateau in Northwest Argentina constitute a classic example of such deposits. These conglomerates may record global climate change, regional or local tectonic uplift, or a combination of those. Here we date 10 ashes collected from within and stratigraphically below the Punaschotter conglomerates using zircon U-Pb geochronology to constrain the timing of conglomerate deposition and sedimentation rates. Structural mapping along transects on the western region of the Fiambalá Basin will constrain the relative timing of deformation in relation to sedimentation. If climate change is the dominant force, the timing of deposition and increased sedimentation rates should correlate along strike in different basins and it should correlate with worldwide climate events. Alternatively, if local structural deformation and tectonic uplift control are dominant factors, sedimentation, and eventually erosion, will correlate poorly or not at all with global climate change and deposition will be syn-tectonic. We expect to find that both tectonics and climate exert control, but that each factor can be evaluated individually. From this study, further insight is gained into the effect that climate and tectonics exert on orogenic growth in a marginal basin setting.
T23D-1656
Rapid Late Miocene Exhumation in the Central Alps, Constrained by (U-Th)/He and Fission Track Thermochronology
Zircon and apatite (U-Th)/He thermochronological data together with apatite fission track analyses are used to explore uplift and exhumation of the western part of the Aar massif in the central Swiss Alps. A total of 27 samples were collected from the surface and underground, from the world's deepest tunnel (Loetschberg NEAT), with an overall elevation difference of almost 2500 m. Zircon (U-Th)/He ages range from 5.5 to 7.6 Ma, apatite fission track ages range from 5.7 to 6.5 Ma and apatite (U-Th)/He ages range from 3 to 5.5 Ma. All zircon age-elevation profiles from three traverses, show distinct brakes in slope that mark a drastic, 10-fold acceleration of exhumation at 6 ± 0.5 Ma ago (from 0.3 km/Ma to 3 km/Ma). The trend of fast exhumation appears to be maintained in the apatite fission track data while apatite (U-Th)/He ages suggest a return to moderate, apparent exhumation rates of 0.5 km/Ma. We propose that the accelerated exhumation may be linked to the Messinian desiccation of the Mediterranean. During that event, Mediterranean sea level dropped locally by as much as 3 km which accelerated erosion in the Alps. Consequently, the erosion in the Alps must have increased. If wedge mechanics are considered, the increased erosional flux reduced the active width of the orogen and, during latest convergence, deformation focused in the internal parts of the Alps, i.e. Aar massif. This interpretation implies a strong and prompt feedback between external forcing and tectonic response of the orogen. The slower exhumation rate apparent from the apatite (U-Th)/He data may reflect a decline in deformation. Alternatively, due to its low closure temperature, this system is prone to resetting by heat advected through hydrothermal circulation. This scenario needs further investigation but abundant hot-water (ca. 50 °C) discharge in the sampled tunnel is not uncommon.
T23D-1657
Composite Faults in the Swiss Alps Formed by the Interplay of Tectonics, Gravitation and Postglacial Rebound: an Integrated Field and Modelling Study
Along the flanks of several valleys in the Swiss Alps, well-preserved fault scarps occur between 1900 and 2400 m altitude, which reveal uplift of the valley-side block relative to the mountain-side block. The height of these uphill- facing scarps varies between 0.5 m and more than 10 m along strike of the fault traces, which usually trend parallel to the valley axes. The formation of the scarps is generally attributed either to tectonic movements or gravitational slope instabilities. Here we combine field data and numerical experiments to show that the scarps may be of composite origin, i.e. that tectonic and gravitational processes as well as postglacial differential uplift may have contributed to their formation. Tectonic displacement may occur as the fault scarps run parallel to older tectonic faults. The tectonic component seems, however, to be minor as the studied valleys lack seismic activity. A large gravitational component, which is feasible owing to the steep dip of the schistosity and lithologic boundaries in the studied valleys, is indicated by the uneven morphology of the scarps, which is typical of slope movements. Postglacial differential uplift of the valley floor with respect to the summits provides a third feasible mechanism for scarp formation, as the scarps are postglacial in age and occur on the flanks of valleys that were filled with ice during the last glacial maximum. Finite-element experiments show that postglacial unloading and rebound can initiate slip on steeply dipping pre-existing weak zones and explain part of the observed scarp height. From our field and modelling results we conclude that the formation of uphill-facing scarps is primarily promoted by a steeply dipping schistosity striking parallel to the valley axes and, in addition, by mechanically weaker rocks in the valley with respect to the summits. Our findings imply that the identification of surface expressions related to active faults can be hindered by similar morphologic structures of non-tectonic origin.
T23D-1658
Thermal History of the Maladeta Pluton: Multiple Thermochronologic Controls on the Burial Heating and Exhumation of the Pyrenean Axial Zone
The Pyrenees are a doubly vergent orogen that formed since the Late Cretaceous as a result of convergence between the European and Iberian plates. During convergence, normal faults were inverted, becoming thrust faults that, in the central Pyrenees, accommodated up to 165 km of shortening. The orogen comprises a central axial zone (AZ) of Hercynian basement, flanked north and south by fold and thrust belts, in turn flanked by the Aquitane and Ebro foreland basins. The AZ is a complex south-vergent duplex that culminates in an antiformal stack of three upper crustal basement thrust sheets. The Maladeta pluton, a Hercynian granitic massif intruded into Silurian-Carboniferous metasediments, lies within the Orri thrust sheet. New K-feldspar 40Ar/39Ar and apatite (U-Th)/He thermochronology combined with previous thermochronology and detailed structural studies of the Maladeta pluton provide a record of post-Hercynian cooling, relative Mesozoic tectonic stability, Late Cretaceous burial and heating, and rapid Cenozoic cooling and denudation. K-feldspar 40Ar/39Ar maximum ages from the highest elevations of the Maladeta pluton (2850m) are ~300 Ma, close to the age of intrusion, suggesting that the Maladeta cooled relatively quickly and remained near the surface or in the upper crust for most of the Mesozoic. This interpretation is consistent with geologic evidence that indicates the Maladeta pluton was part of thinned and extended continental crust, stretched between Iberia and Europe during Mesozoic time, on which marine sediments were deposited unconformably. K-feldspar 40Ar/39Ar multiple diffusion domain (MDD) thermal models from two samples at lower elevations in the Maladeta (1780m and 1400m) record heating beginning at ~80 Ma, reaching maximum temperatures of 270° to 280° C. We interpret this Cretaceous thermal event to represent burial of the Maladeta pluton as the Orri thrust sheet underthrust the Nogueres thrust sheet. This interpretation is consistent with the late Santonian (85.8 - 83.5 Ma) onset of convergence in the Pyrenees recorded by a significant regional unconformity. This Late Cretaceous burial and heating was followed by a period of rapid cooling, beginning at 50-60 Ma, and continuing to at least 30 Ma. Existing lower-temperature thermochronology data (apatite fission track, apatite (U-Th)/He), in addition to our (U-Th)/He ages from the Maladeta indicate extremely rapid cooling at 32-30 Ma due to erosional denudation, with the rate of cooling, and hence erosion, slowing from 30 Ma until ~15 Ma. This lower-temperature thermochronologic record is consistent with constraints from the new K-feldspar MDD models. Thus, we interpret the thermal record of the Maladeta pluton to represent cooling following intrusion during the Hercynian Orogeny, burial during deposition of Permian-Cretaceous sediments in an extensional basin, heating associated with crustal thickening due to thrusting during convergence between Iberia and Europe, and rapid cooling due to erosional denudation during the Cenozoic following the formation of the antiformal stack.
T23D-1659
Late Paleozoic partitioned transpression and heterogeneous extrusion of granitic Monson orthogneiss, Appalachian Orogen, southern New England, USA
Appalachian orogenesis consisted broadly of three Paleozoic collisional phases (Taconian, Acadian, Alleghanian); the kinematics of each phase remain to be fully elucidated. Detailed mapping and structural analysis in central Massachusetts reveal the details of kinematics associated with middle to late Paleozoic convergence of the Bronson Hill (BHT) and Central Maine (CMT) terranes, in that part of the orogen coincident with the greatest shortening perpendicular to strike. Fabrics, structures, and geochronology in the Monson gneiss of the BHT (south-central Mass.), are consistent with continuous Late Paleozoic partitioned dextral transpression, as opposed to multiple tectonic episodes beginning in the Devonian (nappe-backfold-dome stages of previous workers). The Monson is bounded on the east by the dextral Conant Brook high strain zone (HSZ) and on the west by the sinistral Mt. Dumplin HSZ (MDHSZ). Between these HSZs, the Monson is an S>L tectonite with two lineation modes; farther north it is an L>S tectonite with a single lineation mode. Fabrics in the Monson and bounding lithologies across the zone of deformation are characterized by steep to sub-vertical foliations, a heterogeneously distributed bimodal suite of stretching lineations (moderate SSW mode and near-vertical mode, parallel to foliation dip, in Monson, representing coaxial and non-coaxial components; SSW mode only in MDHSZ, representing non-coaxial simple shear), and local closed to isoclinal folds with hinge lines sub-parallel to lineations. Kinematic partitioning, partially unconfined and inclined boundaries that allowed for vertical and lateral extrusion, and oblique simple shear are the primary elements of this zone and suggest heterogeneous flow with an overall triclinic symmetry. We suggest the consistency of fabric across the zone and its boundaries is an artifact of deformation partitioning due to lithologic heterogeneity, and stabilization of the sub-horizontal lineation (manifestation of non-coaxial component) by lateral (northward) extrusion of the Monson. Preliminary U-Pb zircon and monazite geochronology indicates that deformation occurred between 330 to 300 Ma. Lateral mid-crustal orogen-parallel flow is consistent with Late Paleozoic kinematics elsewhere in the BHT.
T23D-1660
Grenville-era Crustal Architecture of Central Australia, and its Importance in Constraining Rodinia Models.
The ca. 1320-1150 Ma Musgravian Orogeny of central Australia is often considered as part of a global system of orogenic belts that transected the super-continent Rodinia during the Grenvillian-era (ca. 1300-1100 Ma), and is commonly interpreted to extend eastwards and connect with contemporaneous orogens in Laurentia. Accurate definition of the architecture and regional kinematics of the Musgrave Province at this time will constrain the tectonics and configuration of Rodinia in the late Mesoproterozoic. Structural understanding of the Musgravian Orogeny is hampered by the development of east-trending crustal scale shear zones during the late Neoproterozoic Petermann Orogeny that have locally reoriented earlier architecture. Distant from these shear zones, where there is greater preservation of early architecture, outcrop is sparse. To analyse Musgravian Orogeny architecture and kinematics, high-fidelity aeromagnetic interpretation and analysis has been applied in a region distant from major Petermann Orogeny shear zones. Our analysis indicates that Musgravian Orogeny architecture is defined by a series of NE trending steeply dipping reverse faults and upright, parallel folds. These structures are truncated by ca. 1180-1150 Ma. Pitjantjatjara Supersuite granitoid intrusions. Throughout the Musgrave Province, Pitjantjatjara Supersuite plutons are located along NE-ENE trending faults, and this structural relationship can be extrapolated to magnetic anomalies observed over the Officer and Amadeus basins: These anomalies possess the distinctive magnetic signature of the Pitjantjatjara Supersuite and are aligned in NE trending chains that connect the Pitjantjatjara Supersuite with ca. 1150 Ma. granites in the Albany Fraser Province and Warumpi Province. Continental scale NE trending structures are therefore interpreted to have been important in controlling the emplacement of ca. 1150 Ma granitoids. In Australia, the distribution of ca. 1150 Ma granitoids defines a continuous and coherent northeast trending orogenic belt connecting the Albany Fraser, Musgrave and Warumpi provinces. The geometry and extent of this orogenic belt precludes a direct connection between the Musgrave Province and contemporaneous orogens in Laurentia. Any model of Australian orogenic activity during the Grenvillian era, must take account of the NE oriented architecture, and intracontinental termination of the orogenic belt. Continental reconfiguration within Australia via the rotation of the South Australian Craton can adequately explain the Grenville-aged architecture of Australia.
T23D-1661
Insights upon upper crustal arhitecture of a subduction zone and its surroundings - Vrancea Zone and Focsani Basin - substantiated by geophysical studies
The DACIA PLAN (Danube and Carpathian Integrated Action on Processes in the Lithosphere and Neotectonics) deep seismic reflection survey was performed in August-September 2001, with the proposed objective of obtaining new information on the deep structure of the external Carpathians nappes and the architecture of Tertiary/Quaternary basin developed within and adjacent to the Vrancea zone, including the rapidly subsiding Focsani Basin. The DACIA-PLAN profile is about 140 km long, having a roughly NW-SE direction, from near the southeast Transylvanian Basin, across the mountainous southeastern Carpathians and their foreland to near the Danube River. A high resolution 2.5D velocity model of the upper crust along the seismic profile has been determined from a tomographic inversion and a 2D ray tracing forward modelling of the DACIA PLAN first arrival data. Peculiar shallow high velocities indicate that pre-Tertiary basement in the Vrancea Zone (characterised by velocities greater than 5.6 km/s) is involved in Carpathian thrusting while rapid alternance, vertically or horizontally, of velocity together with narrowingly contemporary crustal events suggests uplifting. Further to the east, at the foreland basin-thrust belt transition zone (well defined within velocity values), the velocity model suggests a nose of the Miocene Subcarpathians nappe being underlain by Focsani Basin units. A Miocene and younger Focsani Basin sedimentary succession of ~10 km thickness is ascertained by a gradual increase of velocities and strongly defined velocity boundaries.
T23D-1662
Crustal Reflectivity Underneath the Central Scandinavian Caledonides
We discuss data from a ca. 160 km long deep reflection seismic profile in the Swedish part of the Central Scandinavian Caledonides. The seismic data were acquired during 1988-1992, as part of the Central Caledonian Transect. The profile starts within the Precambrian basement to the east of the Caledonian thrust front and extends westwards onto the relatively thin Caledonian thrust sheets, which in this area largely consist of sedimentary rocks of Late Proterozoic or Early Palaeozoic age. One outstanding feature in the seismic image, observed throughout the profile, is the pattern of strong reflectivity down to depths of ca. 15 km. We interpret this reflectivity primarily as due to dolerites intruded in the Precambrian basement. Although the Caledonian thrust sheets cover large parts of the Precambrian basement in the study area, magnetic data reveals structures underneath the thrust sheets. We interpret the dolerites to be located in a homogeneous and highly magnetized granite belonging to the Transscandinavian Igneous Belt, stretching ca. 1400 km from southern Sweden to northern Norway. Based on the pattern of intrusion inferred by the seismic data as well as comparison with ground observations in exposed parts of the Precambrian basement in the region, the dolerites are believed to belong to an 1 Ga system. At the eastern end of the seismic profile, to the east of the thrust front, there are also seismic reflections which can be correlated with dolerites belonging to an older 1.2 Ga intrusion system. The dolerites to the east of the thrust front are clearly visible in magnetic maps. Coinciding with the location of the magnetic TIB granite, the seismic data shows a thinning of the crust, from ca. 50 km to 45 km, and a less distinct reflection Moho. We propose that the decrease in deep reflectivity is related to the emplacement of the TIB granite at 1.85-1.65 Ga. Based also on the observation of weak east-dipping reflections in the middle/lower crust we suggest that extension at ca. 1.0 Ga led to the thinning of the crust and allowed the dolerities to intrude the granites in the upper/middle crust. Later Caledonian compression sheared the granite/dolerite system, resulting in the pattern of reflections observed in the seismic image.
T23D-1663
Geophysical and Thermal Crustal Study of Pull-Apart Basins: The Salton Trough and Death Valley, California Regions
We have constructed new crustal scale models of two unique pull apart basins, the Salton Trough of southwestern California, which is inferred to be an incipient ocean basin, and Death Valley to the east of the Salton Trough, which is a highly extended continental basin. For this work I have used receiver functions, controlled source seismic, gravity and magnetic data to constrain crustal structure. Analysis of gravity data shows that the anomalies in the Salton Trough are deeper than anomalies of Death Valley. My modeling suggests the Moho is 21 km deep south of the Salton Sea and deepens to 33 km in the region west of the Salton Trough, while in Death Valley the Moho is 26 km deep in the central part of the basin and deepens to 32 km on either side. Another significant difference between the two basins is the density of the lower crust, which is 2950 kg/m3 for the Salton Trough and 2750 kg/m3 for Death Valley. Density of the upper crust varies from 2.75 kg/m3 to 2.45 kg/m3 in the Salton Trough and from 2.6 kg/m3 to 2.4 kg/m3 in Death Valley. Sedimentary rocks and meta-sedimentary rocks in Death Valley are thick and reach a depth of 15 km, while in the Salton Trough the depth of sedimentary rocks and meta-sedimentary rocks is 8-9 km. The rate of deposition is higher in Death Valley than in the Salton Trough which is formed from magmatism in the lower crust and sedimentation in the upper crust. Rising of upper mantle material causes uplifting, thinning, and crustal extension (rifting) in the central part of the Salton Trough south of the Salton Sea, and in the southern part of Death Valley. Magnetic anomalies are shallow in both regions. The anomalies in Death Valley show higher relief (@ 420 nT, compared to 250 nT) than in Salton Trough. In the Salton Trough the magnetic anomalies are almost flat with some exceptions in the marginal areas. Magnetic data were processed to calculate the Curie point depth, wavelength of the magnetic bodies and depth of the magnetic bodies. Curie point depth in the Salton Trough ranges from 14-22 km, which is consistent with other geothermal studies and measurements. Curie point depth measurements for Death Valley range from 12- 20 Km.
T23D-1664
3D Thermal/Mechanical Evolution Of The Plate Boundary Corner In SE Alaska
The St Elias orogen of southeast Alaska forms part of an actively deforming plate boundary corner. The corner accommodates the transition from a strike-slip lateral boundary to a convergent normal boundary. Oblique convergence of the Yakutat microplate into the corner generates early stage tectonic characteristics associated with other corner systems (e.g. Himalayan Eastern Syntaxis). In combination with the high relief, the extreme erosive processes of the region redistribute crustal material, partition tectonic strain, and influence the advection of deep crustal material. The evolution of the convergent corner is investigated using 3D numerical models and sandbox analog models. Preliminary model results indicate the deformation partitions into a narrow two-sided orogen along the lateral boundary. The pattern transitions into a wider zone of shortening bounded by inboard and outboard directed thrusts along the frontal boundary. The inclusion of erosion boundary conditions leads to nascent tectonic aneurysm behavior, involving increased strain localization and focused vertical advection of deep crustal material. Thermal models, using the 3D velocity field from these mechanical solutions, show a vertical deflection (towards the surface) of isotherms beneath the eroding region. Sensitivity of the aneurysm behavior is related to the efficiency of the imposed erosion rate (i.e. greater erosion rates led to greater bedrock uplift rates). Higher erosion rates are localized within zones containing major glacier systems in SE Alaska: Bering Glacier, Bagley Icefield, Malaspina Glacier, and Seward Glacier. Combined thermal/mechanical solutions identify the glacier valleys as rheological weakspots, defined by localized strain and differential advection of deep crustal material.
T23D-1665
Exhumation and shortening distribution in the Taiwan orogen: insights from thermomechanical modeling
The Taiwan orogen has long been regarded as a case example for studying exhumation and erosion processes in association with mountain building. In the recent years, the increasing number of thermochronometric data (mainly ZFT and AFT ages) has allowed to better understanding the deep-seated tectonic processes. For instance, using thermomechanical wedge modelling, up to 50% of underplating has been proposed to explain the observed distribution of FT ages. So it would appear that a large amount of materials added to the orogen originated in more deeper and ductile parts of the crust. The nature of this additional flux (velocity, mechanism of deformation) is however poorly constrained in the current thermomechanical model. Our concern is to use a fully- coupled visco-elasto-plastic themomechanical numerical model to reproduce the observed FT ages and long- term distribution of shortening. To this aim we use the numerical code PARA(O)VOZ based on F.L.A.C. (Fast Lagrangian Analysis of Continua) algorithm. Our results show that the particular distribution of shortening across the Taiwan belt as well as the rapid exhumation and thermal conditions in the hinterland are well accounted for by two superimposed flows of upper and lower crustal rocks decoupled from the subducting Eurasian mantle.
T23D-1666
The 3-D density structure of Taiwan Area by using converted tomographic model
Taiwan situated at the collision between the Eurasian plate and the Philippine Sea plate with different subduction polarity in the north and the south Taiwan. The complex tectonic background maked varied geologic structures beneath Taiwan. In order to obtain more detail and more significant information about the structure beneath Taiwan, it would be better to join two or three kinds of geophysical data to investigate the subsurface formation beneath Taiwan. Most usually, use the tomography from seismic data to derive the velocity and the velocity gradient between the interfaces. In this study, we converted the P wave velocity to density by using three different tomography results in Taiwan (Rau, 1995, Ma, 1996 and Kim, 2005). The method we use to convert the P wave velocity and density is referencd to Nafe & Drake, 1957 and Barton, 1986. We used the converted density model as initial model to calculate theoretical bouguer anomaly and inverse the proper density structure. We compared with the final inversed density results of these three different velocity-density initial models. While these three density results might not fit data very well, we still can have an idea which one is better than others. In realistic world, the more physical parameters are thought about, the more reality can be achieved. In this study, the result is expected for satisfying both tomography and density model.